The temperature dependence of devices may be an important factor affecting their precision and their basic operation. This is particularly important in the case where the temperature is a control parameter for device operation. Many temperature-controlled devices have been developed so far for various applications, including the field of optical telecommunications.
The in-guide modulation of light propagation (light remains in the original waveguide while being transformed) appears to be the best solution for the fabrication of low loss optical components. Its main underlying principle is the use of the evanescent part of the guided light to affect it dynamically. The traditional way to do this is the removal and replacement of a part of the cladding of the guide and the dynamic change of the optical properties of that part of cladding, particularly in the area where the evanescent field propagates. In the case of the variable optical attenuator (VOA) application, the materials used must have relatively high sensitivity to the external excitation (e.g., thermal) to provide sufficiently high refractive index changes (compared to the refractive index of the core of the guide) and corresponding light attenuation.
Composite materials, such as polymers, have been proposed as good candidates for such applications. This class of materials is well adapted for low-cost fabrication techniques and is easy to manipulate in comparison with complex and costly operations needed to fabricate semiconductor components such as micro electro mechanical systems (MEMSs).
One of the frequently used properties of polymers is their high sensitivity to external excitation signals, such as temperature, which reduces the electrical power consumption of the device. Thus, U.S. Pat. No. 6,240,226 to Presby et al. for a “Polymer material and method for optical switching and modulation” describes a planar Mach-Zender interferometer employing a section of thermo-optic polymer cladding in one branch, the polymer cladding having an index of refraction that varies with temperature. The temperature of the section of polymer cladding is adjusted, for example by a planar electrical heater, to cause a corresponding change in the refractive index of the polymer and consequently in the phase of the light flowing through the waveguide core bounded by the polymer cladding to achieve a desired switching or modulation of light. The device is well adapted to be fabricated in a cost effective (printed circuit) way.
In FIG. 1 (PRIOR ART), the cross section of a thermally controlled polymer planar waveguide is schematically presented. Light may be guided in the direction perpendicular to the drawing surface in the core area 102 (a channel guide created in the layer 101), typically fabricated on a substrate layer 103. In the case where light is guided in the core 102, the layers 101 and 103 are applied as low refractive index claddings for the core 102. To achieve a dynamic control of light propagation, an electro-conductive layer 104 (e.g., a thin film electrode) is fabricated on the top of the layer 101, close to the core area 102. The typical operation of this device is based on the control of the refractive index of the core 102 (and/or of the layer 101) by means of an electrical current flowing through the electrode 104 that releases heat. The released heat leads to a controllable refractive index change and consequently to a phase shift of the guided light and thus to its variable attenuation. The attenuation is typically achieved via the destructive interference of a Mach-Zender interferometer.
An important problem persists with the above-mentioned architectures (polymer materials and planar asymmetric geometries) that is related to the relatively high shrinkage and stress induced optical birefringence of the applied thermo-optic polymers, which results into undesired polarization dependant loss (PDL). Indeed, during the fabrication of the polymer layer, the shrinkage-induced stress (due to the asymmetry of boundary conditions of the polymer layer) and corresponding birefringence may create strong PDL. In addition, during the operation (e.g., thermal switch) of the device, the release of heat (by means of the electrode 104) in an asymmetric way usually leads to the creation of additional stress, birefringence and corresponding PDL. That is why many additional efforts were deployed to reduce the PDL. For example, the work of H. Kawashima, N. Matsubara, K. Nara, and K. Kashihara, entitled “Ultra-Low PDL PLC based Thermo-Optic VOA” and published in ECOC-IOOC 2003 Proceedings—Vol. 3, Paper We3.2.4, pp. 498–499, describes significant PDL reduction by creating specific air-channels 105 (see FIG. 1) in the polymer layer 101 that allow the lateral expansion of the polymer film and reduction of the stress induced birefringence.
However, the coupling of light from planar photonic circuits to standard optical fibers (or vice versa) is usually accompanied by high optical losses due to the guided light mode mismatch when propagating in fibers (cylindrical geometry) and the above-described planar geometries. The use of all-fiber devices would significantly decrease the insertion losses of the system and facilitate the assembly tasks. That is why the idea of using the approach of thermo-optic polymer cladding has attracted much attention in the fiber geometry. Thus, U.S. Pat. No. 6,466,729 to Wagoner et al. entitled “Controllable fiber optic attenuators employing tapered and/or etched fiber sections” describes controllable fiber optic attenuators and attenuation systems for controllably extracting optical energy from a fiber. In contrast with the interferential attenuation described above, a simple leakage of optical radiation is traditionally used in the fiber geometry. Thus, a portion of the optical fiber is etched or tapered, thereby providing a side surface, coated with the thermo-optic material, through which optical energy can be extracted by means of the thermal control of the refractive index of the coated material by a heater/cooler element.
U.S. Pat. No. 6,483,981 to Krahn et al. entitled “Single-channel attenuators” describes a VOA comprising a controllable heating/cooling source in contact with a polymer composition, wherein the controllable heating/cooling source provides a controllable stimulus to the polymer composition to change the temperature thereof and to adjust the refractive index of the polymer composition.
The heating electrode configuration is a paramount issue in these inventions. Several geometries have been proposed to increase the functionality of those electrodes, using for example, metal heaters with tapered thickness and corresponding electrical resistance, see for example the work of B. J. Eggleton, et al. published in Journal of Lightwave Technology, vol. 18, pp. 1418–1432, 2000. The heat release being proportional to the electrode resistance, this device allows refractive index control with a tunable gradient. Multiple electrodes (with uniform and tapered thickness) along with temperature sensors have also been proposed to achieve a better control of spectral properties of the fiber. See for example the work of J. A. Rogers and B. J. Eggleton, entitled “Temperature Stabilized Operation of Tunable Fiber Grating Devices that Use Distributed on-fiber thin film heaters”, Electronics Letters, vol. 35, pp. 2052–2053, November 1999.
In spite of the noticeable progress achieved (for example very low insertion losses), the fiber attenuators still have significant drawbacks. Two key remaining difficulties are the heating electrode asymmetry and the inhomogeneities (non uniformities, such as low precision of its quantity and geometrical shape) of the thermo-optically controllable polymer that is replacing a part of the fiber cladding. In fact, very often, the controllable material initially is a monomer solution, which is then polymerized by thermal and/or UV curing. Non uniformities and bubbles are then formed if a solid (“closed”) container is used to support the initial liquid solution. Otherwise, asymmetric boundary conditions are generated if the initial solution is just cast on the fiber surface. Those non uniformities and asymmetry of the final polymer form generate stress-induced optical birefringence and strong PDL, which is a major problem. Finally, the asymmetry of the control electrode and corresponding heat release during the operation of the device usually aggravates the PDL problem.
The fabrication of fiber-based attenuation devices and the precise control of their properties thus remain a difficult task. The methods and elements described above are complex, not efficient and costly. The corresponding fabrication methods and operation principles do not allow the simple control of device parameters, in particularly the PDL.